Thermal energy storages play an important role in the improvement of the stability of power supply networks and for increasing the energy efficiency. There are different types of thermal energy storages depending on how the energy is stored (using heat capacity of a material or using phase change enthalpy or even using chemical reaction enthalpy). Generally, these thermal storages are facilities that are filled with thermal storage elements, which have the ability to be heated up and keep a certain temperature. The thermal storage elements are heated (charged) through a working flow of fluid (gas, liquid or a mixture thereof), which has a higher temperature than the thermal storage elements. The stored energy can be recovered through a flow of fluid (with the same or a different composition), which has a lower temperature than the thermal storage elements.
When the charged thermal energy storage is at a standstill period, i.e. a period where no charging or discharging flow of working fluid is fed to the storage, a flow of fluid may still be created within the storage because of natural convection phenomena (temperature gradients). These streams can both stress the materials (mechanical stress) and create a non-uniform temperature profile in the storage. For these reasons, the creation of such streams caused by natural convection should be avoided.
Known attempts at solving this problem involve the use of horizontal or vertical plates within a thermal energy storage to limit convection within the structure of thermal storage elements during standstill.
In the case of vertical plates, i.e. plates that are perpendicular to the flow direction, the plates may be placed alternately in the upper and lower part of the storage. Such a pattern reduces the convection in the storage but at the same increases the pressure losses during charging and discharging of the storage. Moreover, this pattern cannot provide a uniform temperature distribution in the thermal energy storage during the standstill period. After some time, the hot fluid will be collected in the upper part of the structure, while colder fluid will be present in the lower part of the structure. Such temperature gradients may cause stress and potentially damage to the thermal storage elements.
In the case of horizontal plates, i.e. plates that are parallel to the flow direction, a number of plates are installed in the main storage between layers of thermal storage elements in order to limit the free volume for the fluid, within which it can move. Thereby, natural convection is limited within the structure of thermal storage elements, as a flow of fluid is prevented by the plates. However, in cases where an empty space exists at the front and/or at the back (relative to the direction of flow) of the thermal storage structure, natural convection may still occur within such empty space(s), thereby effectively by-passing the horizontal plates. The result is that, also in this case, after some time of standstill, the hot fluid will be collected in the upper layers of thermal storage elements, while colder fluid will be collected in the lower layers of thermal storage elements.
Accordingly, there may be a need for an improved thermal energy storage which does not suffer from the above-mentioned drawbacks of the known storages.